Battery protection and zero-volt battery recovery system for an implantable medical device

ABSTRACT

Circuitry useable to protect and reliably charge a rechargeable battery, even from a zero-volt state, is disclosed, and is particularly useful when employed in an implantable medical device. The circuit includes two charging paths, a first path for trickle charging the battery, and a second path for charging the battery at relatively higher currents. A passive diode is used in the first trickle-charging path which allows trickle charging even when the battery voltage is too low for reliable gating, while a gateable switch (preferably a PMOS transistor) is used in the second higher-current charging path when the voltage is higher and the switch can therefore be gated more reliably. A second diode between the two paths ensures no leakage to the substrate through the gateable switch during trickle charging. The load couples to the battery through the switch, and preferably through a second switch specifically used for decoupling the load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/625,892, filed Jun. 16, 2017 (allowed), which isa continuation application of U.S. patent application Ser. No.13/099,591, filed May 3, 2011 (now U.S. Pat. No. 9,687,663), which is acontinuation application of U.S. patent application Ser. No. 11/550,872,filed Oct. 19, 2006 (now U.S. Pat. No. 7,962,222), which in turn wasbased on U.S. Provisional Patent Application Ser. No. 60/748,240, filedDec. 7, 2005. These applications are hereby incorporated by reference intheir entireties.

FIELD OF THE INVENTION

The present invention relates generally to circuitry and techniques forbattery protection and zero-Volt battery recovery preferably for use inan implantable medical device such as an implantable stimulator device,or more specifically an implantable pulse generator.

BACKGROUND

Implantable stimulation devices generate and deliver electrical stimulito body nerves and tissues for the therapy of various biologicaldisorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a Spinal Cord Stimulation (SCS) system, such asthat disclosed in U.S. Pat. No. 8,606,362, which is incorporated hereinby reference in its entirety.

Spinal cord stimulation is a well-accepted clinical method for reducingpain in certain populations of patients. An SCS system typicallyincludes an Implantable Pulse Generator (IPG) or Radio-Frequency (RF)transmitter and receiver, electrodes, at least one electrode lead, and,optionally, at least one electrode lead extension. The electrodes, whichreside on a distal end of the electrode lead, are typically implantedalong the dura of the spinal cord, and the IPG or RF transmittergenerates electrical pulses that are delivered through the electrodes tothe nerve fibers within the spinal column. Individual electrode contacts(the “electrodes”) are arranged in a desired pattern and spacing tocreate an electrode array. Individual wires within one or more electrodeleads connect with each electrode in the array. The electrode lead(s)exit the spinal column and generally attach to one or more electrodelead extensions. The electrode lead extensions, in turn, are typicallytunneled around the torso of the patient to a subcutaneous pocket wherethe IPG or RF transceiver is implanted. Alternatively, the electrodelead may directly connect with the IPG or RF transceiver. For examplesof other SCS systems and other stimulation systems, see U.S. Pat. Nos.3,646,940 and 3,822,708, which are hereby incorporated by reference intheir entireties. Of course, implantable pulse generators are activedevices requiring energy for operation, such as is provided by animplanted battery or an external power source.

As should be obvious, an IPG needs electrical power to function. Suchpower can be provided in several different ways, such as through the useof a rechargeable or non-rechargeable battery or through electromagnetic(EM) induction provided from an external charger, or from combinationsof these and other approaches, which are discussed in further detail inU.S. Pat. No. 6,553,263 (“the '263 patent”), which is incorporatedherein by reference in its entirety. Perhaps the favorite of theseapproaches is to use a rechargeable battery in the IPG, such as alithium-ion battery or a lithium-ion polymer battery. Such arechargeable battery can generally supply sufficient power to run an IPGfor a sufficient period (e.g., a day or more) between recharging.Recharging can occur through the use of EM induction, in which EM fieldsare sent by an external charger to the IPG. Thus, when the battery needsrecharging, the patient in which the IPG is implanted can activate theexternal charger to percutaneously (i.e., through the patient's flesh)charge the battery (e.g., at night when the patient is sleeping orduring other convenient periods).

The basics of such a system are shown in FIG. 1, which is largelydescriptive of salient contents of the '263 patent. As shown, the systemcomprises in relevant part the external charger 208 and IPG 100. Asnoted, a coil 279 in the charger 208 produces an EM field 290 capable ofpercutaneous transmission through a patient's flesh 278. The externalcharger 208 may be powered by any known means, such as via a battery orby plugging into a wall outlet, for example. The EM field 290 is met atthe IPG 100 by another coil 270, and accordingly an AC voltage isinduced in that coil 270. This AC voltage is turn is rectified to a DCvoltage at a rectifier 682, which may comprise a standard bridgecircuit. (There may additionally be data telemetry associated with theEM field 290, but this detail is ignored as impertinent to the presentdisclosure). The rectified DC voltage is in turn sent to a chargecontroller 684, which operates generally to regulate the DC voltage andto produce either a constant voltage or constant current output asnecessary for recharging the battery 180. The output of the chargecontroller 684, i.e., how aggressively the charge controller charges thebattery 180, is dependent on the battery voltage, Vbat, as will beexplained in further detail later. (The charge controller 684 can alsobe used to report the battery 180's charge status back to the externalcharge 208 via back telemetry using coil 270, as disclosed in the '263patent; however, because this function is not particularly relevant tothis disclosure, it is not further discussed).

The output of the charge controller 684 is in turn met by two switches701, 702 which respectively prevent the battery 180 from over-chargingor over-discharging. As shown, these transistors are N-channeltransistors, which will be “on,” and thus capable of connecting thecharge controller 684's output to the battery 180 when their gates arebiased. Control of these gates is provided by a battery protectioncircuit 686, which receives the battery current and voltage, Ibat andVbat, as control signals, again as will be explained in further detaillater. For example, whenever the battery 180 exhibits too high avoltage, the battery protection circuit 686 will turn off the gate ofthe over-charging transistor 701 to protect the battery from furthercharging. A fuse positioned between the transistors 701, 702 and thebattery 180 may also be used to further protect the battery from veryhigh current events (not shown). The battery 180 is coupled to one ofseveral loads in the IPG 100, such as the electrode stimulationcircuitry, i.e., the circuits the battery 180 ultimately powers. Thebattery 180 is coupled to such loads through a load switch 504, whichcan isolate the battery 180 from the load to protect one from adverseeffects of the other. This load switch 504 is preferably part of thecharge controller 684, which may comprise its own integrated circuit,although this is not strictly necessary.

As discussed in the above-referenced '263 patent, the charging circuitry684 can charge the battery 180 in different ways, depending on thestatus of the battery voltage, Vbat. Without reiterating the contents ofthat disclosure, such selective charging of the battery 180 isbeneficial for safely charging the battery, particularly when alithium-ion-based battery is used. Essentially, this safe chargingscheme charges the battery 180 with smaller currents when the batteryvoltage Vbat is significantly depleted, and charges with higher currentswhen the battery voltage is still undercharged but at higher, saferlevels.

Consider an embodiment in which Vbat=4.2V represents a nominal voltagefor the battery 180. When Vbat<2.5V, the charge controller 684 will“trickle” charge the battery 180 with a low level current, e.g., Ibat=10mA. As the battery charges and as Vbat increases, higher chargingcurrent can be used. For example, once Vbat>2.5V, a charging current ofIbat=50 mA may be set by the charge controller 684. Once the nominalvoltage of 4.2V is approached, the charge controller 684 may continue tocharge the battery 180 by providing a constant voltage instead ofconstant current on its output, which as charging continues is manifestin a gradual decay of the battery current. The relationship between Vbatand Ibat during battery charging is graphically illustrated in FIG. 2.Of course, these various current and voltage values are merelyexemplary, and other parameters may be suitable depending on the systemat hand. Also, more than two levels of charging current (e.g., 10 mA, 25mA, and 50 mA) can be used in stair-step fashion.

As noted earlier, the battery protection circuit 686 prevents thebattery from potential damage during charging by disconnecting thebattery from the charge controller 684. Specifically, Vbat exceeds asafe value (e.g., greater than 4.2V), then the over-charging transistor701 is disabled by the battery protection circuit 686 to block furthercharging. Likewise, if the battery voltage is less than a predeterminedvalue and if Ibat exceeds a predetermined value, over-dischargetransistor 702 is disabled to prevent discharging of the battery. Whiledisclosed as controlling two transistors 701, 702, the batteryprotection circuit 686 may control a single disabling protectiontransistor which functions to disable the battery 180 during bothover-charging and over-discharging. Load switch 504 may be similarlycontrolled to isolate the components to protect them from adversevoltages and currents.

While the charging and protection circuitry of FIG. 1 is suitable, itsfunctionality may be hampered at extremely low battery voltages. As the'263 patent explains, this is because the battery protection circuit 686is powered by the battery voltage, Vbat, and hence when Vbat isextremely low (e.g., approaching zero Volts), the battery protectioncircuitry 686 may not function as desired. In this regard, note thatwhen Vbat is extremely low, and thus when the battery 180 is in need ofcharging, the battery protection circuit 686 needs to be able to turntransistors 701 and 702 on, else the charging controller 684 will not beable to pass a charging current, Ibat, to the battery. However, whenIbat is low, the battery protection circuit 686 may have difficultygenerating a sufficient voltage to turn on the gates of the N-channeltransistors 701 and 702. Specifically, the battery protection circuitry686 must be able to produce a gate voltage for the transistors that isgreater than Vgs (i.e., the potential difference between the gate andsource of the transistors). In short, the battery protection circuitryneeds to be able to produce a gate voltage which exceeds a thresholdvoltage (Vt) of the transistors given the source voltages apparent atthe transistors. If Vbat is below this threshold voltage, the batteryprotection circuit 686 may not be able to produce a suitably-high gatevoltage to turn transistors 701 and 702 on.

Should this occur, the battery 180 cannot be charged, even though Vbatis low and hence the battery 180 is very much in need of charging. Inother words, the charging and protection circuitry in FIG. 1 ispotentially susceptible to failure at when Vbat is extremely low, i.e.,at zero Volts or near-zero Volts. In a worst case this would mean thatthe IPG 100 is unrecoverable, and if implanted in a patient, may requirethe drastic step of surgical removal and replacement of the device. Butthis is unfortunate, because patients in which IPGs are implanted cannotnecessarily be relied upon to diligently charge their implanted devices,and hence the risk of a depleted, unrecoverable battery is very real.

As a result, improved circuitry and techniques for protection andzero-Volt recovery for batteries in implantable medical devices would bebeneficial. Such solutions are provided herein.

SUMMARY

Circuitry useable to protect and reliably charge a rechargeable battery,even from a zero-volt state, is disclosed, and is particularly usefulwhen employed in an implantable medical device. The circuit includes twocharging paths, a first path for trickle charging the battery at arelatively low current when the battery voltage is below a threshold,and a second path for charging the battery at relatively higher currentsthat the battery voltage is above a certain threshold. A passive diodeis used in the first trickle-charging path which allows trickle chargingeven when the battery voltage is too low for reliable gating, while agateable switch (preferably a PMOS transistor) is used in the secondhigher-current charging path when the voltage is higher and the switchcan therefore be gated more reliably. A second diode between the twopaths ensures no leakage to the substrate through the gateable switchduring trickle charging. The load couples to the battery through theswitch, and preferably through a second switch specifically used fordecoupling the load.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will be moreapparent from the following more particular description thereof,presented in conjunction with the following drawings wherein:

FIG. 1 shows a prior art system comprising an external charger forcharging an implantable pulse generator (IPG), and shows the chargecontroller and battery protection aspects of the IPG.

FIG. 2 graphically shows the relationship between battery voltage andbattery current during charging of the battery as dictated by the chargecontroller of FIG. 1.

FIG. 3 shows a block diagram that illustrates exemplary implantable,external, and surgical components of a spinal cord stimulation (SCS)system that employs an implantable stimulator device in accordance withthe present invention.

FIG. 4 shows various components of the SCS system of FIG. 1.

FIG. 5 shows a block diagram that illustrates the main components of oneembodiment of an implantable stimulator device in which the inventioncan be used.

FIG. 6 shows a block diagram that illustrates another embodiment of animplantable stimulator device in which the invention can be used.

FIG. 7 is similar to the system illustrated in FIG. 1, but includesimproved battery protection and zero-Volt recovery circuitry inaccordance with an embodiment of the invention.

FIG. 8 shows the various sensors used in conjunction with the improvedprotection and zero-Volt recovery circuitry of FIG. 7.

FIG. 9 shows the main switch used in the improved protection andzero-Volt recovery circuitry of FIG. 7, and shows parasitic currentpaths that can exist.

DETAILED DESCRIPTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims and their equivalents.

Before discussing the battery protection and zero-Volt recovery aspectsof the invention that is the focus of this disclosure, the circuitry,structure, and function of an implantable stimulator device in which thedisclosed circuitry and technique can be used is set forth forcompleteness with respect to FIGS. 3-6. The disclosed implantablestimulator device may comprise implantable pulse generator (IPG), orsimilar electrical stimulator and/or electrical sensor, that may be usedas a component of numerous different types of stimulation systems. Morespecifically, the description that follows relates to use of theinvention within a spinal cord stimulation (SCS) system as an exemplaryembodiment. However, it is to be understood that the invention is not solimited. Rather, the invention may be used with any type of implantableelectrical circuitry that could benefit from improved battery protectionand zero-Volt recovery techniques. For example, the present inventionmay be used as part of a pacemaker, an implantable pump, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical or deepbrain stimulator, or in any other stimulator configured to treat urinaryincontinence, sleep apnea, shoulder sublaxation, etc. Moreover thetechnique can be used in non-medical and/or non-implantable devices orsystems as well, i.e., in any device or system in which zero-Voltbattery recovery and/or protection is necessary or desirable.

Turning first to FIG. 3, a block diagram is shown that illustrates thevarious components of an exemplary SCS system in which the invention maybe used. These components may be subdivided into three broad categories:implantable components 10, external components 20, and surgicalcomponents 30. As seen in FIG. 3, the implantable components 10 includean implantable pulse generator (IPG) 100, an electrode array 110, and(as needed) a lead extension 120. The extension 120 may be used toelectrically connect the electrode array 110 to the IPG 100. In anexemplary embodiment, the IPG 100, described more fully below inconnection with FIG. 5 or 6, may comprise a rechargeable, multi-channel,telemetry-controlled, pulse generator housed in a roundedhigh-resistivity titanium alloy case to reduce eddy current heatingduring the inductive charging process. The IPG 100 may provideelectrical stimulation through a multiplicity of electrodes, e.g.,sixteen electrodes E₁ through E₁₆, included within the electrode array110.

In this regard, the IPG 100 may include stimulating electrical circuitry(“stimulating electronics”), a power source, e.g., a rechargeablebattery, and a telemetry system, the latter of which is particularlyrelevant to embodiments of the disclosed invention. Typically, the IPG100 is placed in a surgically-made pocket either in the abdomen, or justat the top of the buttocks. It may, of course, also be implanted inother locations of the patient's body. Once implanted, the IPG 100 isconnected to the lead system, comprising the lead extension 120, ifneeded, and the electrode array 110. The lead extension 120, forexample, may be tunneled up to the spinal column. Once implanted and anytrial stimulation period is complete, the lead system 110 and leadextension 120 are intended to be permanent. In contrast, the IPG 100 maybe replaced if it fails.

As seen best in FIG. 4, and as also illustrated in FIG. 3, the electrodearray 110 and its associated lead system typically interface with theimplantable pulse generator (IPG) 100 via the lead extension system 120just mentioned. The electrode array 110 may also be connected to anexternal trial stimulator 140, through the use of a percutaneous leadextension 132 and/or an external cable 134. The external trialstimulator 140 typically includes the same or similar pulse generationcircuitry as does the IPG 100, and is used on a trial basis, e.g., for7-10 days, after the electrode array has been implanted and prior toimplantation of the IPG 100, to test the effectiveness of thestimulation that is to be provided.

Still with reference to FIGS. 3 and 4, and as noted earlier, a hand-heldprogrammer (HHP) 202 may be used to control the IPG 100 via a suitablenon-invasive communications link 201, e.g., an RF link. Such controlallows the IPG 100 to be turned on or off, and generally allowsstimulation parameters, e.g., pulse amplitude, width, and rate, to beset by a patient or clinician within prescribed limits. The HHP 202 mayalso be linked with the external trial stimulator 140 through anotherlink 205′, e.g., an infra red link. Detailed programming of the IPG 100is preferably accomplished through the use of an external clinician'sprogrammer (CP) 204 (FIG. 3), which may also be hand-held and which maybe coupled to the IPG 100 directly via link 201 a or indirectly throughthe HHP 202. An external charger 208, non-invasively coupled with theIPG 100 through link 290, e.g., an inductive link, allows energy storedor otherwise made available to the charger 208 to be coupled into therechargeable battery 180 housed within the IPG 100, as explained in theBackground.

Turning next to FIG. 5, a block diagram is shown that illustrates themain components of one embodiment of an implantable pulse generator(IPG) 100 in which embodiments of the invention may be used. As seen inFIG. 5, the IPG includes a microcontroller (μC) 160 connected to memorycircuitry 162. The μC 160 typically comprises a microprocessor andassociated logic circuitry, which in combination with control logiccircuits 166, timer logic 168, and an oscillator and clock circuit 164,generate the necessary control and status signals which allow the μC 160to control the operation of the IPG in accordance with a selectedoperating program and stimulation parameters. (A “microcontroller” asused herein should be understood as any integrated device capable ofprocessing signals in the IPG, including traditional microcontrollers,microprocessors, or other signal processors, including those that areapplication-specific, such as ASIC chips).

The operating program and stimulation parameters are telemetered to theIPG 100, where they are received via antenna 250 (which may include acoil 170 and/or other antenna components), processed, e.g., viaRF-telemetry circuitry 172, and may be stored, e.g., within the memory162. As noted earlier, the RF-telemetry circuitry 172 demodulates thesignal it receives from the HHP 202 or CP 204 to recover the operatingprogram and/or the stimulation parameters. More specifically, signalsreceived by the antenna 250 are passed through the transmit/receiveswitch 254 to amplifiers and filters 258. From there, the receivedsignals are demodulated (262) using Frequency Shift Keying (FSK)demodulation for example, and the data is then sent to themicrocontroller 160 for processing and/or eventual storage. WhenRF-telemetry circuitry 172 is used to transmit information to the HHP202 or CP 204 to report in some fashion on its status, themicrocontroller 160 sends relevant data to transmission drivers 256,where the carrier is modulated by the data and amplified fortransmission. The transmit/receive switch 254 would then be set tocommunicate with the transmission drivers 256, which in turn drive thedata to the antenna 250 to be broadcast.

The microcontroller 160 is further coupled to monitoring circuits 174via bus 173. The monitoring circuits 174 monitor the status of variousnodes or other points 175 throughout the IPG 100, e.g., power supplyvoltages, current values, temperature, the impedance of electrodesattached to the various electrodes E₁ . . . E_(N), and the like.Informational data sensed through the monitoring circuit 174 may be sentto a remote location external to the IPG (e.g., a non-implantedlocation) through telemetry circuitry 172 via coil 170.

The operating power for the IPG 100 may be derived from a rechargeablepower source 180, which may comprise a lithium-ion or lithium-ionpolymer battery, for example, as discussed earlier. The rechargeablebattery 180 provides an unregulated voltage to power circuits 182. Thepower circuits 182, in turn, generate the various voltages 184, some ofwhich are regulated and some of which are not, as needed by the variouscircuits located within the IPG 100. In a preferred embodiment, thebattery 180 is charged by an electromagnetic field created by anexternal portable charger 208 (FIGS. 1, 3) as already noted. When placednear the IPG 100 (e.g., centimeters away), an electromagnetic fieldemanating from the portable charger 208 induces a current in chargingcoil 270 (even through a patient's skin). This current is then rectifiedand regulated to charge the battery 180, as explained in the Background.Further associated with the charging circuitry is charging telemetrycircuitry 272, which is used for example by the IPG 100 to report backto the portable charger 208 when the battery is full, and thus whenportable charger can be shut off.

In one exemplary embodiment, any of the N electrodes may be assigned toup to k possible groups or “channels.” In one preferred embodiment, kmay equal four. Moreover, any of the N electrodes can operate, or beincluded in, any of the k channels. The channel identifies whichelectrodes are selected to synchronously source or sink current tocreate an electric field in the tissue to be stimulated. Amplitudes andpolarities of electrodes on a channel may vary, e.g., as controlled bythe HHP 202. External programming software in the CP 204 is typicallyused to set parameters including electrode polarity, amplitude, pulserate and pulse width for the electrodes of a given channel, among otherpossible programmable features.

The N programmable electrodes can be programmed to have a positive(sourcing current), negative (sinking current), or off (no current)polarity in any of the k channels. Moreover, each of the N electrodescan operate in a bipolar mode or multipolar mode, e.g., where two ormore electrode contacts are grouped to source/sink current at the sametime. Alternatively, each of the N electrodes can operate in a monopolarmode where, e.g., the electrode contacts associated with a channel areconfigured as cathodes (negative), and the case electrode (i.e., the IPGcase) is configured as an anode (positive).

Further, the amplitude of the current pulse being sourced or sunk to orfrom a given electrode contact may be programmed to one of severaldiscrete current levels, e.g., between □0 to □10 mA in steps of 0.1 mA.Also, the pulse width of the current pulses is preferably adjustable inconvenient increments, e.g., from 0 to 1 milliseconds (ms) in incrementsof 10 microseconds (μs). Similarly, the pulse rate is preferablyadjustable within acceptable limits, e.g., from 0 to 1000 Hz. Otherprogrammable features can include slow start/end ramping, burststimulation cycling (on for X time, off for Y time), and open or closedloop sensing modes.

The stimulation pulses generated by the IPG 100 may be charge balanced.This means that the amount of positive charge associated with a givenstimulus pulse is offset with an equal and opposite negative charge.Charge balance may be achieved through coupling capacitors Cx, whichprovide a passive capacitor discharge that achieves the desiredcharge-balanced condition. Alternatively, active biphasic ormulti-phasic pulses with positive and negative phases that are balancedmay be used to achieve the needed charge balanced condition.

In short, the IPG 100 is able to individually control the currents atthe N electrodes. Controlling the output current Digital-to-AnalogCurrent (DAC) circuitry 186 using the microcontroller 160, incombination with the control logic 166 and timer logic 168, allows eachelectrode contact to be paired or grouped with other electrode contacts,including the monopolar case electrode, to control the polarity,amplitude, rate, pulse width and channel through which the currentstimulus pulses are provided.

As shown in FIG. 5, much of circuitry included within the IPG 100 may berealized on a single application specific integrated circuit (ASIC) 190.This allows the overall size of the IPG 100 to be quite small, andreadily housed within a suitable hermetically-sealed case. The IPG 100may include N feedthroughs to allow electrical contact to beindividually made from inside of the hermetically-sealed case with the Nelectrodes that form part of the lead system outside of the case.

As noted earlier, in use, the IPG 100 may be placed in a surgically-madepocket, e.g., in the abdomen or just at the top of the buttocks, anddetachably connected to the lead system (comprising optional leadextension 120 and electrode array 110). While the lead system isintended to be permanent, the IPG 100 may be replaced should it fail.

The telemetry features of the IPG 100 allow the status of the IPG to bechecked. For example, when the HHP 202 and/or the CP 204 initiate aprogramming session with the IPG 100, the capacity of the battery istelemetered so that the external programmer can calculate the estimatedtime to recharge. Any changes made to the current stimulus parametersare confirmed through back-telemetry, thereby assuring that such changeshave been correctly received and implemented within the implant system.Moreover, upon interrogation by the external programmer, allprogrammable settings stored within the implant system 10 may beuploaded to one or more external programmers.

Turning next to FIG. 6, a hybrid block diagram of an alternativeembodiment of an IPG 100′ that may be used with the invention isillustrated. The IPG 100′ includes both analog and digital dies, orintegrated circuits (ICs), which may be housed in a singlehermetically-sealed rounded case having, for instance, a diameter ofabout 45 mm and a maximum thickness of about 10 mm. Many of the circuitscontained within the IPG 100′ are identical or similar to the circuitscontained within the IPG 100, shown in FIG. 5. The IPG 100′ includes aprocessor die, or chip, 160′, an RF telemetry circuit 172′ (typicallyrealized with discrete components), a charger coil 270′, a rechargeablebattery 180′, battery charger and protection circuits 272′, 182′, memorycircuits 162′ (SEEPROM) and 163′ (SRAM), a digital IC 191′, an analog IC190′, and a capacitor array and header connector 192′.

The capacitor array and header connector 192′ include sixteen outputdecoupling capacitors, as well as respective feed-through connectors forconnecting one side of each decoupling capacitor through thehermetically-sealed case to a connector to which the electrode array110, or lead extension 120, may be detachably connected.

The processor 160′ may be realized with an application specificintegrated circuit (ASIC), field programmable gate array (FPGA), or thelike that comprises a main device for full bi-directional communicationand programming. The processor 160′ may utilize an 8086 core (the 8086is a commercially-available microprocessor available from, e.g., Intel),or a low power equivalent thereof, SRAM or other memory, two synchronousserial interface circuits, a serial EEPROM interface, and a ROM bootloader 735. The processor die 160′ may further include an efficientclock oscillator circuit 164′, and (as noted earlier) mixer andmodulator/demodulator circuitry implementing the QFAST RF telemetrymethod. An analog-to-digital converter (A/D) circuit 734 is alsoresident on the processor 160′ to allow monitoring of various systemlevel analog signals, impedances, regulator status and battery voltage.The processor 160′ further includes the necessary communication links toother individual ASICs utilized within the IPG 100′. The processor 160′,like all similar processors, operates in accordance with a program thatis stored within its memory circuits.

The analog IC (AIC) 190′ may comprise an ASIC that functions as the mainintegrated circuit that performs several tasks necessary for thefunctionality of the IPG 100′, including providing power regulation,stimulus output, and impedance measurement and monitoring. Electroniccircuitry 194′ performs the impedance measurement and monitoringfunction.

The analog IC 190′ may also include output current DAC circuitry 186′configured to supply current to a load, such as tissue, for example. Theoutput current DAC circuitry 186′ may be configured to deliver up to 20mA aggregate and up to 12.7 mA on a single channel in 0.1 mA steps.However, it will be noted that the output current DAC circuitry 186′ maybe configured to deliver any amount of aggregate current and any amountof current on a single channel, according to one exemplary embodiment.

Regulators for the IPG 100′ supply the processor and the digitalsequencer with a voltage. Digital interface circuits residing on theanalog IC 190′ are similarly supplied with a voltage. A programmableregulator supplies the operating voltage for the output current DACcircuitry 186′. The coupling capacitors C_(X) and electrodes E_(X), aswell as the remaining circuitry on the analog IC 186′, may all be housedwithin the hermetically sealed case of the IPG 100. A feedthrough pin,which is included as part of the header connector 192′, allowselectrical connection to be made between each of the coupling capacitorsC_(N) and the respective electrodes E₁, E₂, E₃, . . . , or E₁₆.

The digital IC (DigIC) 191′ functions as the primary interface betweenthe processor 160′ and the output current DAC circuitry 186′, and itsmain function is to provide stimulus information to the output currentDAC circuitry 186′. The DigIC 191′ thus controls and changes thestimulus levels and sequences when prompted by the processor 160′. In anexemplary embodiment, the DigIC 191′ comprises a digital applicationspecific integrated circuit (digital ASIC).

With the basic structure of an implantable stimulator understood, focusnow shifts to a detailed description of the battery protection andzero-Volt recovery aspects that are the focus of this disclosure. It isagain worth noting that while particularly useful when implemented inimplantable medical devices in which the problem of zero-Volt batteryrecovery is unique, the disclosed techniques can benefit any device orsystem in which zero-Volt recovery is beneficial. Thus, disclosure inthe context of an implantable medical device should be understood asmerely exemplary.

Improved battery protection and zero-Volt recovery circuitry 500 isshown in FIG. 7. Because many of the components are similar tocomponents discussed earlier with respect to FIG. 1, they are labeledwith the same element numerals, even if differing slightly in functionas noted herein. Protection and zero-Volt recovery circuitry 500 ispreferably formed on an integrated circuit, but may also comprisediscrete components. Moreover, circuitry 500 may be integrated with thecharge controller 684 or with other integrated circuits in the IPG 100,the level of integration and/or combination of functions being merely adesign choice.

Briefly, protection and zero-Volt recovery circuitry 500 comprises in apreferred embodiment two distinct charging paths: one (designated bynode “Trickle”) for trickle changing, and another (designated by node“Plus”) used for normal charging. At least one diode 501 (a passivedevice, unlike a transistor which must be actively gated) intervenesbetween node Trickle and the battery voltage, Vbat. (If more than onediode is used, they would be serially connected, although this is notshown in FIG. 7 for simplicity). A main switch 503 (preferably aP-channel MOS transistor) intervenes between node Plus and Vbat, whichis controlled by a main switch control circuit 505, explained in furtherdetail with respect to FIG. 8. Load switch 504 intervenes between nodePlus and the load (designated by node “Vdd”), and is generally similarin function to the load switch of FIG. 1. However, it should be notedthat coupling the battery 180 to the load requires that both switches503 and 504 be closed, with main switch control 505 controlling theformer, and the charge controller 684 for example controlling thelatter. Additionally, a diode 502 intervenes between nodes Plus andTrickle. (In other embodiments, diode 502, like diode 501, may actuallycomprise a chain of serially-connected diodes, although FIG. 7 shows asingle diode for simplicity).

Protection and zero-Volt recovery circuitry 500 basically supports andcontrols two operative modes: a charging mode and discharging mode.

The discharging mode is implicated when the battery 180 is coupled tothe load, e.g., during normal operation, through main switch 503 andload switch 504. In discharge mode, the circuit 500 can sense a shortcircuit, i.e., from node Plus or Vdd to ground and/or excessive currentdraw, either of which evidences a problem with the IPG 100. When eithercondition is detected, the main switch 503 is turned off by main switchcontrol circuit 505 to prevent the battery 180 from being drained, andwill remain off until the external charger 208 turns it back on, a pointdiscussed in further detail later.

The charging mode can further be classified into two sub-modes: atrickle charging mode and a normal charging mode, similar to thatdiscussed with respect to FIGS. 1 and 2. When the battery 180 voltage,Vbat, is sensed by the charge controller 684 to be less than the 2.5V,the circuit is in trickle charging mode. As will be seen, during tricklecharging, current (Itrickle; approximately 10 mA) is passed from thecharge controller 684 through node Trickle to charge the battery up to2.5V. Once Vbat equals 2.5V, the charge controller 684 switches to thenormal charging mode, thus passing a larger current (Inormal;approximately 50 mA) through node Plus to the battery. When the battery180 is finally fully charged, the main switch 503 is turned off toisolate the battery 180 from the charge controller.

As noted, the main switch 503 is controlled on and off by main switchcontrol circuitry 505, and it is useful at this point to briefly explainhow the logic in this circuitry 505 reacts to open and close the mainswitch 503. FIG. 8 thus show the various sensing circuits used to openand close the switch 503, not shown in FIG. 7 for clarity. Whilerelevant to protection, one skilled in the art will realize that thesevarious sensors may also be integrated with the charge controller 684.

Shown in FIG. 8 are four sensors: a short circuit sensor 510, a batteryvoltage sensor 512, an excess current sensor 516, and a main switchsubstrate sensor 520. Each sensor receives as input either the batteryvoltage, Vbat, or the voltage at node Plus as shown. Reed switch 522,preferably a discrete component separate from other integratedcircuitry, can be used to freely disable the main switch 503, and thusterminate charging or discharging of the battery 180, for any number ofother reasons.

Short circuit sensor 510 monitors the voltage at node Plus. If thisvoltage falls below an acceptable value (e.g., 0.8V), sensor 510 infersthat a short circuit is present between Plus and ground (or between Vddand ground if load switch 504 is on), and so directs the main switchcontrol circuitry 505 to disable main switch 503, thus isolating thebattery 180 to prevent it from draining. For safety reasons, shouldshort circuit sensor 510 direct the main switch control circuitry 505 todisable the main switch 503, it is preferable that the main switch 503stay permanently open until the external charger 208 (FIG. 1) is used toraise node Plus to an acceptable operating voltage. Of course, whetherthe external charger 208 will be capable of resetting main switch 503ultimately depends on removal of the condition or “glitch” which causedthe initial short circuit.

Battery voltage sensor 512 senses the voltage of the battery 180, Vbat,and can be used to inform the charge controller 684 of this value, e.g.,so that the charge controller knows when to transition between trickleand normal charging. Battery voltage sensor 512 is also useful to assesswhether Vbat is too high (e.g., greater than 4.2V), and if so, toactivate self discharge circuit 514 to lower the voltage to a properlevel. During normal operation, the battery voltage sensor 512 directsthe main switch control 505 to close main switch 503 should Vbat bewithin normal operating parameters, e.g., between 2.5V and 4.2V, andotherwise directs control 505 to disable switch 503. The various voltagelevels of interest to the battery voltage sensor 512 (e.g., 2.5V, 4.2V),may be trimmed to adjust their values to account for process variationsvia a multi-bit bus (not shown).

Excess current sensor 516, like short circuit sensor 510, is used todisconnect the battery 180 under conditions of high current draw. In apreferred embodiment, sensor 516 senses excessive current by measuringthe voltage drop across the main switch 503, i.e., from node Plus toVbat. Knowing the “on” resistance of the main switch 503 (preferablybetween 0.12 to 0.19 ohms when Vbat=3.6V), should the voltage dropsuggest an excessive high current draw (e.g., greater than 400 mA), theshort circuit sensor 510 directs the main switch control 505 to disablethe main switch 503.

The main switch substrate sensor 520 monitors the polarity of currentflow (charge or discharge) across the main switch 503 and ties theN-well of the main switch 315 to the higher of Plus or Vbat to preventcurrent loss to the substrate, as described further below.

The various sensors illustrated in FIG. 8 may be constructed usingstandard reference circuits, such as bandgap voltage reference circuits,voltage dividers, differential amplifiers, comparators, etc. Becausesuch sensors circuits are well known and may take several differentforms as one skilled in the art will appreciate, they are not discussedfurther.

The main switch 503 is implemented with a PMOS transistor residing in anN well, such as is shown in cross-section in FIG. 9. It should be notedthat the source and drain regions of the main switch 503 (which areotherwise symmetrically fabricated as one skilled in the art willappreciate, and hence arbitrarily named in FIG. 9) will be subject todifferent voltages depending on whether the battery 180 is being chargedor discharged. For example, during normal charging, node Plus is broughthigh due to biasing from the charge controller 684; while beingdischarged, Vbat is brought relatively high by the battery 180.

Since the source and drain regions of the main switch 503 are subject tothese different voltage polarities, the N well potential (node “Bias” inFIG. 9) is tied to the higher of potential of the source or drain ofmain switch 503 at all times, rather than simply always having the Nwell biased to Vbat as would be more typical for a PMOS transistor. Thisprevents unwanted current draw to the substrate.

To illustrate this problem, consider normal charging of battery. Whencharging commences, the provision of current (i.e., voltage) at nodePlus may be relatively high when compared with Vbat, i.e., depending onVbat's current level of charge. If Vbat is coupled to the N well at nodeBias, a parasitic PNP bipolar transistor (540; FIG. 9) could turn on,with the result that current meant to trickle charge the battery 180would be routed to the substrate of the main switch 503. Suchinadvertent siphoning of the current would at least slow the charging ofthe battery 180, or if significant, could prevent charging of thebattery 180 altogether. However, this problem is not simply fixed bytying the N well (Bias) to node Plus, because during discharging, theparasitic effect could be realized at the other of the source/drain ofthe switch 503: that is, during discharge, Vbat could be higher thanPlus, which could turn on the parasitic PNP bipolar transistor at theother terminal (542; FIG. 9), causing current otherwise useable by theload to be inadvertently drained to the substrate. To address thisproblem, as noted, the N well (node Bias) is biased to the higher of thesource or drain nodes of main switch 503 via a polarity control circuit,such as the main switch substrate sensor 520 discussed above (see FIG.8). The design of such a polarity control circuit is easily made bythose of skill in the art and can be made in many different ways.

With this overview of the protection and zero-Volt recovery circuitry500 in hand, attention can know be focused on how the circuit 500operates to protect and charge the battery 180, even from a completelyzero-Volt state.

In this regard, and as noted earlier, note from FIG. 7 that the tricklecharging path (node Trickle; current Itrickle) is separated from normalcharging path (node Plus; current Inormal). Zero-Volt recovery is thedriving factor for separating these two charging paths. By separatingthe Trickle and Plus nodes, the trickle charging current (Itrickle) canbe prevented from flowing into the Plus node, and hence bipolartransistor parasitics in the main switch 503 are not implicated. At thesame time, because the trickle current bypasses the main switch 503,potential uncertainty in the status of the switch at low voltages is oflittle concern.

However, what is of concern is the possibility of current leakagethrough node Plus to the substrate. This is alleviated in one embodimentby holding node Plus at a suitable voltage level during tricklecharging. Specifically, in a preferred embodiment, during tricklecharging, node Plus is tied to Vbat. This is accomplished in oneembodiment through the use of diode(s) 502. The function of diode(s) 502is to match the voltage drop across diode(s) 501 during trickle chargingto keep the voltage at Plus the same as Vbat so that trickle chargecurrent does not leak to the substrate through switch 503. In otherwords, if 502 were not present, the voltage at Plus could be below thebattery and 503 could leak to the substrate.

Although diodes 501 and 502 are shown as single diodes in FIG. 7,different numbers of diodes can be used. In a preferred embodiment, twodiodes in series are used for diodes 501 and 502. Moreover, it should berealized that diodes 501, 502 can be realized as transistors in whichone of the source and drain are tied to the substrate (well), as is wellknown. As such, “diode” as used herein should be understood as includingsuch structures, and any other structures capable of one-way currenttransmission.

With Plus held to Vbat during trickle charging, even if the N well islikewise biased to Vbat, the parasitic bipolar transistor 540 (FIG. 9)cannot turn on due the zero-Volt potential difference between the Plusand the well. Moreover, pursuant to this biasing scheme, all junctionnodes of the main switch 503 are held to the same potential, i.e.,source=drain=N well=Vbat. The result is that no current can flow throughmain switch 503, regardless of whatever potential is present on the gateof the transistor. That being said, when it is desired for the mainswitch 503 to be off, it may still be preferable to bias the gate ofmain switch 503 to the higher of the two voltages apparent at its sourceand drain regions, just as the N well is biased as explained earlier.(Similar circuitry could be used for both of these functions). This evenfurther helps to ensure that the PMOS main switch 503 will not conductduring trickle charging.

Thus, in contrast to the circuit of FIG. 1 as discussed in theBackground, low values for Vbat, which affected the ability to turn ontransistors 701, 702, become irrelevant, and zero-Volt recovery of thebattery is realized through node Trickle. This is true even if thevarious sensors, such as battery voltage sensor 512 (FIG. 8), cannotoperate reliably at lower values of Vbat, because trickle charging canreliability occur regardless of what such sensors would otherwiseindicate to the main switch control 505, and how the main switch control505 would bias the gate of the main switch 503. With the Plus nodeeffectively prevented from drawing current during trickle charging,trickle charging can be accomplished by having the charge controller 684providing a significantly high bias to node Trickle to overcome theforward threshold on diode 501 to produce the desired trickle current,Itrickle.

Once the battery 180 has been trickled charged as just described,eventually Vbat will raise to a level at which normal charging can takeplace, e.g., at 2.5V. By way of review, monitoring of Vbat for thiscross-over condition is the function of battery voltage sensor 512 ofFIG. 8, which, like main switch control 505 will start functioningreliably once Vbat is charged to a suitably high level (e.g., 2.0V).Thus, whereas during trickle charging the gate voltage of main switch503 was irrelevant because the voltage conditions in the source, drain,and well prevented current flow, during normal charging, voltages havebeen charged to a suitable level that the main switch control 505 andother sensors reporting to it can now reliably turn on the switch 503 topermit the flow of current (Inormal) from the charge controller 684 tonode Plus, and ultimately to the battery 180. Because the voltage atnode Plus will be relatively high during normal charging, diode 502 willbe reversed-biased, thus preventing the flow of current to node Trickle.

In summary, protection and zero-Volt recovery circuitry 500 is capableof both protecting the battery, and charging the battery 180 even from azero-Volt condition. Protection is present by the ability to isolate thebattery 180 from both the load via load switch 504 and from the chargecontroller via main switch 503 and via diode 501 (which will preventbattery discharge back to the charge controller 684). Such protectiondoes not hamper the circuitry 500 from being charged. Instead, throughthe provision of two separate charging paths, Plus and Trickle, thebattery can be charged through one path (Trickle) without concern thatthe protection circuitry will inhibit low-level charging when thebattery is at low voltages. Once suitable charged, the other path (Plus)is used to charge the battery through the protection circuitry (e.g.,main switch 503) to nominal voltages. Thus, the battery and load areprotectable from adverse voltage and current conditions, and the batterycan be fully recovered. As noted earlier, this is especially importantwhen the circuitry 500 is incorporated in an implantable medical devicesuch as an IPG 100, for failure to recover a fully depleted batterymight otherwise warrant surgical extraction of the device.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the literal and equivalent scope of the invention setforth in the claims.

What is claimed is:
 1. An implantable medical device, comprising: arechargeable battery configured to provide a battery voltage to power aload; a first charging path comprising first switching circuitryconfigured to control a connection between a first charging currentissued from a first node in the first charging path and the battery; andsecond switching circuitry configured to control a connection betweenthe first node and the load, controller circuitry configured to closethe first switching circuitry in a first mode of operation when thefirst charging current is being issued from the first node to thebattery, and configured to close the first and second switchingcircuitries in a second mode of operation when the battery voltage isbeing used to power the load.
 2. The implantable medical device of claim1, further comprising a plurality of electrodes configured to contact atissue of a patient, wherein the load comprises Digital-to-Analogcontroller circuitry configured to provide a current to one or more ofthe electrodes.
 3. The implantable medical device of claim 1, furthercomprising a second charging path configured to provide a secondcharging current to the battery, wherein the second charging pathcomprises at least one first diode forward biased from a second node inthe second charging path to the battery.
 4. The implantable medicaldevice of claim 3, further comprising at least one second diode forwardbiased from the second node to the first node.
 5. The implantablemedical device of claim 4, wherein the at least one first diode isconfigured to bias the first node to the battery voltage when the secondcharging current is being provided to the battery.
 6. The implantablemedical device of claim 1, wherein the first switching circuitryconsists of a PMOS transistor.
 7. The implantable medical device ofclaim 6, wherein a well of the PMOS transistor is biased to a higher ofa source or a drain of the PMOS transistor.
 8. The implantable medicaldevice of claim 6, wherein a well of the PMOS transistor is coupled tothe battery terminal.
 9. The implantable medical device of claim 6,wherein a well of the PMOS transistor is biased to a voltage of thefirst node.
 10. The implantable medical device of claim 1, furthercomprising a short circuit sensor configured inform the controllercircuitry to open the first switching circuitry if a voltage at thefirst node falls below a threshold to isolate the battery.
 11. Theimplantable medical device of claim 1, further comprising a batteryvoltage sensor configured to inform the controller circuitry to open thefirst switching circuitry if the battery voltage is below a firstthreshold or above a second threshold higher than the first threshold.12. The implantable medical device of claim 11, further comprising aself discharge circuit, wherein the battery voltage sensor is configuredto activate the self discharge circuit to lower the battery voltage ifthe battery voltage is greater than the second threshold.
 13. Theimplantable medical device of claim 11, wherein the controller circuitryis configured to close the first switching circuitry in the first modeof operation when the battery voltage is between the first and secondthresholds.
 14. The implantable medical device of claim 1, furthercomprising an excess current sensor configured to measure a voltage dropbetween the first node and the battery voltage, and configured to informthe controller circuitry to open the first switching circuitry if thevoltage drop indicates a current draw through the first switchingcircuitry that is greater than a threshold.
 15. A method for rechargingand using a battery in an implantable medical device, comprising:closing first switching circuitry to pass a first charging current froma first node through the first switching circuitry to recharge thebattery in a first mode of operation; and closing the first switchingcircuitry and a second switching circuitry to connect the batterythrough the first and second switching circuitries to a load during asecond mode of operation, wherein the second switching circuitry isconnected between the first node and the load.
 16. The method of claim15, wherein the implantable medical device comprises a plurality ofelectrodes configured to contact a tissue of a patient, and wherein theload comprises Digital-to-Analog controller circuitry configured toprovide a current to one or more of the electrodes.
 17. The method ofclaim 15, wherein the first switching circuitry is closed when a batteryvoltage of the battery is greater than a first threshold.
 18. The methodof claim 17, further comprising passing a second charging current from asecond node to recharge the battery when the battery voltage is greaterthan or equal to zero and less than or equal to the first threshold. 19.The method of claim 15, further comprising opening the first switchingcircuitry if a battery voltage of the battery is greater than a secondthreshold.
 20. The method of claim 15, further comprising opening thefirst switching circuitry if a current through the first switchingcircuitry is above a threshold.